The present invention relates to an image forming apparatus such as a printer, copying machine, facsimile machine or the like.
Referring first to FIG. 7, there is shown a structure of a transfer portion of an image forming apparatus such as a printer according to a background art.
The photosensitive drum 1 is rotated in the direction indicated by an arrow R1, and a toner image is formed thereon by charging means, exposure means and developing means (unshown). The toner image is transferred onto a recording material such as paper by a transfer roller (transferring means).
The transfer roller 2 is contacted to the photosensitive drum 1 to form a transfer nip T therebetween. The transfer roller 2 is supplied with a high voltage (transfer bias) from a high voltage source (transfer bias application voltage source). The voltage applied to the transfer roller 2 is controlled through a predetermined sequence by control means 4, and the current flowing through the transfer roller 2 is detected by current detecting means 5. Upstream of the transfer nip T along a feeding path 6 of the recording material, there is provided a sensor 7 for detecting leading and trailing edges of the recording material fed in the direction indicated by an arrow Kp. The distance from the sensor 7 to the transfer nip T is set to L.
FIG. 8 shows a voltage applied to the transfer roller 2 when the printing (image formation) is carried out on one recording material. The abscissa represents time, and ordinate represents presence or absence of the recording material and the voltage (transfer bias).
The time 0 corresponds to the time when the user instructs the printing operation. The control means 4 adjusts the voltage such that current detected by the current detecting means 5 is a predetermined value. The current is 2 xcexcA, for example. Since the resistance value of the transfer roller 2 varies depending on the ambient conditions under which the apparatus is used and on the transfer rollers, the voltage is adjusted to provide a proper current for image transfer operation. The voltage required for flowing the current suitable for the image transfer is designated by C in FIG. 8. The high voltage source 3 is controlled at time A which is t1 after C, such that voltage D is applied from the point of time (time a) when the sensor 7 detects the presence of the sheet after the recording material is fed to the point of time at which the recording material reaches the transfer nip T. At the time when the sensor 7 detects the absence of the recording material (time b) is deemed as being the point of time at which the trailing-edge of the recording material passes, and the high voltage source 3 is controlled at time B t2 thereafter, such that voltage restores to C prior to the trailing edge of the recording material passes the transfer nip T. When the printing is finished, the output of the voltage is stopped at proper timing.
The voltage is raised during the recording material being material transfer nip T, in order to compensate the reduction of the current which otherwise occurs because of the existence of the recording material in the transfer nip T. The voltage D is not applied from the initial stage, because the high voltage across the transfer nip T without the recording material in the need may damage the photosensitive drum 1. In order to prevent such damage, the setting is selected so as to assure that voltage D is reached only after the recording material come into the transfer nip T, even when the rising time F is minimum.
However, due to the increasing printing speed of the image forming apparatus and compatibility with a variety of media, there is a tendency that voltage D required for the printing is increasing with a result of the increasing rising time F from the setting of the high voltage to the actual arrival at the voltage D. The timing at which the output of the high voltage source 3 is changed is selected such that voltage D is not applied in the absence of the recording material in the transfer nip T, and therefore, there is a liability that output of the high voltage source 3 is not sufficiently high to the point of time when the leading edge of the recording material reaches the transfer nip T. In such a case, the toner is not sufficiently transferred from the photosensitive drum to the recording material after the leading-edge of the recording material to the arrival of the voltage from the high voltage source 3 at voltage D (image defect of transfer void). If, on the other hand, voltage D is too high, the toner jumps so vigorously at the recording material that image defect occurs due to the so-called scattering of the toner.
Accordingly, it is a principal object of the present invention to provide an image forming apparatus in which the rising of the voltage is made quick to avoid the transfer void without damaging the image bearing member.
It is another object of the present invention to provide an image forming apparatus in which scattering of the image attributable to the too high voltage, is effectively prevented.
According to an aspect of the present inventions, there is provided an image forming apparatus comprising an image bearing member; a transferring member for transferring a toner image from said image bearing member onto a recording material at a transfer portion; voltage applying means for applying a voltage to said transferring member; wherein said voltage applying means is capable of applying to the transferring member a first voltage, a second voltage higher than the first voltage and a third voltage higher than the second voltage; wherein said voltage applying means applies the first voltage to the transferring member, then the second voltage prior to the recording material reaching the transfer portion, and then the third voltage after the recording material reaching the transfer portion.
According to another aspect of the present invention, there is provided an image forming apparatus comprising an image bearing member; a transferring member for transferring a toner image from said image bearing member onto a recording material at a transfer portion; voltage applying means for applying a voltage to said transferring member; wherein said voltage applying means is capable of applying to the transferring member a first voltage, a second voltage higher than the first voltage and a third voltage higher than the second voltage: said voltage applying means is operable in a first mode in which said voltage applying means applies to said transferring member the first voltage, then the second voltage prior to the recording material reaching said transfer portion, and then the third voltage after the recording material reaching said transfer portion, and in a second mode in which said voltage applying means applies to said transferring member the first voltage, then the second voltage prior to the recording material reaching said transfer portion, and then the second voltage after the recording material reaching the transfer portion, too; and switching means for switching between the first mode and the second mode.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.